Year: 2000
Type: article
Abstract: Reaction of brucite [Mg(OH)z] with COZ produces various metastable hydrous magnesium carbonates, which will eventually mature to the only thermodynamically stable phase, ma,gnesite (MgC03).Natural analog systems suggest that, at ambient temperatures, the transformation from a key metastable phase, hydromagnesite [4MgCOseMg(OH)zc4HzO], requires hundreds of years.A suite of experiments were conducted to elucidate mechanisms of transformation of hydromagnesite to magnesite by varying ionic strength and concentrations of magnesium and sulfate, and reacting hydromagnesite at temperatures between 110, 150 and 200°C.Arrhenius equations were used to extrapolate rate information collected at elevated temperature down to 25°C.An "induction" period, an interval of minimal transformation, was observed in all experiments.At 25"C, the induction period is estimated to require 18 to 200 years, with longer times attributed to higher magnesium concentrations.Once the induction period is completed, and more rapid transformation begins, the corresponding "half-lives" of hydromagnesite are about 4.7 and 73 years.Activation energies for the rapid transformation period were estimated to be between 81 and 100 kJ/mol.Transformation rates increased with increased ionic strength and decreased magnesium concentration.Sulfate concentration indirectly affected rates, by reducing magnesium activity through complexation.Two mechanisms for the transformation are supported.In brines with low magnesium concentration, hydromagnesite dehydration with concomitant formation of brucit e and magnesit e is favored.In brines with high magnesium concentration, a hydromagnesite dissolution -magnesite precipitation process is favored.Those mechanisms are also supported by Transmission Electron Microscopy (TEM) observations of reaction products.
Authors: Zhang, Pengchu, Anderson, Howard L., Kelly, John W., Krumhansl, James L., Papenguth, Hans W.
Institution: Sandia National Laboratories
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W1518133764